Objective - To determine the extended family history of diabetes or autoimmune diseases in families with and without children having type 1 diabetes.

Research design and methods – 300 case families and 381 control families were interviewed using structured questionnaires.

Results - The proportion of case children having at least one relative with type 1 diabetes outside the nuclear family was higher than that of control children (50.3% vs. 31.8%; p<0.001). The proportions of cases and controls having relatives with type 2 diabetes or gestational diabetes were similar. Other autoimmune diseases occurred more frequently among the cases (9.7% vs. 1.1%, p<0.001), in the case nuclear families (22.0% vs. 12.9%, p=0.002) and in relatives outside the case nuclear family (72.0% vs. 62.2%, p=0.007).

Conclusion - Type 1 diabetes and autoimmune diseases not only cluster in the nuclear families of children with type 1 diabetes but are also overrepresented in their extended families.